Medical imaging systems presently are committed to using rare earth permanent magnets made from alloys of neodymium iron boron (Nd-Fe-B), such as Fe.sub.14 Nd.sub.2 B. The main polarizing field in this system is provided by two very large neodymium iron boron (NdFeB) permanent magnets, with an iron yoke used for the return path.
The standard techniques for preparation of the highest performance NdFeB permanent magnets build on the process developed for samarium cobalt magnets, such as SmCo.sub.5 magnets. The alloy is prepared, crushed to a fine powder, oriented in a magnetic field, pressed, sintered, annealed, machined, magnetized, and used. This is done with powders and may require no melting. While there are many steps in this process, the overall cost is dominated by the first step, preparation of the alloy. For SmCo.sub.5 magnets, the Reduction-Diffusion (R-D) process has become the most economical approach used for preparation of the alloy. In this process cobalt powder, calcium granules, and rare earth oxide powder are blended together and reacted under hydrogen at 1150.degree. C. The calcium reduces the samarium oxide, and the samarium metal diffuses into the cobalt. After cooling, the excess calcium and calcium oxide are removed from the reacted product by hydrating with wet nitrogen, followed by washing with water and dilute acid. The principal cost advantage for this approach is realized by starting samarium as an oxide rather than as a pure metal.
Variations of the Reduction-Diffusion process have been applied to of neodymium iron boron (NdFeB) permanent magnets. It was found that alloy composition control and the leaching step are more difficult and expensive with of neodymium iron boron (NdFeB) than samarium cobalt (SmCo.sub.5) magnets, limiting somewhat the commercial usefulness of these variations for of neodymium iron boron (NdFeB). As a result, separate reduction and melting steps are primarily used for preparation of the of neodymium iron boron (NdFeB) alloy commercially. This approach requires very expensive high performance vacuum melting furnaces. Thus, a need is created for a lower cost method to make the of neodymium iron boron (NdFeB) alloy needed for the permanent magnet.